An enhancement of neoplastic transformation in vitro due to neutron dose protraction at doses less 1 Gy (100 rads or 100 cGy) has been reported for the Argonne National Laboratory (ANL) JANUS reactor-produced fission neutrons. Since such an effect contradicts current theories of the effects of densely-ionizing radiation at the single cell level, we recently tried to reproduce some of these observations under similar exposure conditions, but measured no dose-rate effect. This study aims at: (1) establishing the dose-response relationship in terms of dose- rate of fission neutrons for neoplastic effects, (2) determining the relative biological (RBE) of fission neutrons relative to 60Co Gamma- rays, and its dependence on neutron dose and dose rate, (3) testing a leading hypothesis put forward as a possible explanation of the dose- rate effects reported for JANUS neutrons. We will use C3H/10T1/2 cells derived from mouse-embryo cells in which transformation is assessed by scoring piled-up foci on the top of a confluent monolayer. Experiments will be performed at 37 degrees C with actively growing cultures. In the experiments addressing the first two aims, cells will be exposed to 0.05 to 0.9 Gy of fission neutrons or to 0.25 to 1. 2 Gy of Gamma-rays at 0.0025 Gy/min or at high dose rates, 0.3 Gy/min (neutrons) or 1 Gy/min (gammas). High and low dose rate irradiations at fixed doses of neutrons or Gamma-rays will be performed on the same day. In experiments addressing the third aim, cells will be exposed to doses of fission neutrons from 0.075 to 0.9 Gy protracted over irradiation time period T = 0.5, 1,2, or 3 h. The effect of fission neutrons dose rate will be measured by comparing the number of transformants induced by protracted or acute exposures (T = 45 sec to 3 min) at a fixed dose. Exposures will be provided by the TRIGA Reactor and Cobalt Facilities at the Armed Forces Radiobiology Institute, Bethesda, Maryland. Radiation field from the AFRRI reactor has the dose-mean lineal energy 71 keV/um.